It has been demonstrated that pre-treatment of mouse lymphoid cells with anti-Ia (I-region-associated) sera and complement abrogates the T cell proliferative response to Con A and lentil lectin, but not PHA. Genetic studies have shown that the Ia determinants expressed on Con A reactive cells are coded for by a least two different subregions of the I-locus. Anti-Ia sera and complement have also been shown to prevent the generation of CML (cell-mediated lympholysis) suppressor cells induced by 5-day culturing of normal spleen cells.